At first I want to clarify that I do know that there are naturally poisonous pla

Question

At first I want to clarify that I do know that there are naturally poisonous plants and plants that can hold radioactive stuff or toxic material from the environment.

What I mean by "diseases" (I guess there is a special term for that in English) is Bovine Spongiform Encephalopathy and similar serious issues caused by eating animals, by that you cannot develop when consuming plants.

As far as I know there are no diseases causes by eating fruit and vegetables (of course, except parasites if you don't wash it properly and an infected cow or bunny pooped on it ;D)

Is it because the parasites and diseases infecting plants are so specialised for their "body-type and genetics" that they cannot operate inside an animal body?

Could somebody explain that to me in more depth why by eating plants you cannot get a disease (except allergic reaction/intolerance or toxicities as mentioned earlier)?

Explanation / Answer

If you mean if there are viruses, pathogenic bacteria or fungi that may cause diseases both in plants and in animals, the response would be that this is very unlikely (But even so, some fungi and bacteria could do it if some circumstances are given). This is due to some key factors:

First of all, the philogenetic distance between plants and animals is huge. Diseases transmitted by the consumption of animals are called zoonosis, and obviously it is more likely to catch a zoonosis by eating pork than eating shrimps or clams. While it's true that certain parasites have different hosts and use them to transmit themselves (for instance, many platyhelmintha uses molluscs or insects to spread themselves towards mammalian or avian hosts) parasites than have only one kind of host usually are adapted to live inside them, and they usually are unable to infect different organisms. Very few parasites can infect a human and a starfish, and even less can infect at the same time a human and a tomato plant.

Second, and related to the first, plant phisiology differs completely from animal phisiollogy. Plants usually defend themselves by their natural barriers (cuticles, cell walls), hypersensitive response (programmed cell death in order to limit the spread of a pathogen), isolation and/or outgrowing of the pathogen and acumulation of chemicals. Animals main mechanism is immune system. There are very different strategies and very few organisms can effectively avoid all of them at the same time. In the case of viruses, they would even have to face the fact that animals and plants have huge genetic differences.

Third even if they could, the pathogen will face competition against other pathogens that would be more specialized. Bacteria and fungi can be really hostile towards their microfoes, using a wide array of strategies. Pur hypothetical pathogen would be very unspecific, so they would have a hard time against more focused microbes. It should be noted, however, that unspecialized organisms usually have a great advantage if the environment have some perturbation, and they also are more resilient toward stress. Many plant pathogens are generalistic organisms that attacks when the plant is harmed or stressed. The same applies to many oportunistic infections in animals.

Despite all of this, there are some diseases that should be mentioned. Some fungi of the Clavyceps genera are plan pathogens, but they produce toxins than can affect the nervous system of animals, causing an intoxication called ergotism (the most famous of this chemicals is the lisergic acid, also known as LSD). Other fungi can produce toxins while they grow in plant tissue. Moreover, there are many fungi that can grow both in plant tissue and as animal parasites (mainly arthropods and nematodes), but in this case the plant pathognesis usually occurs only when high fungal concentration is achieved or the plant is already damaged. While it's not a real infection, plant pathogens can cause allergic reactions. Finally, phytopathogenic fungi and some bacteria can cause infections in wounds or mucoses in individuals with a compromised immune system, but in this case the source seems to be the spores in the air.

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